# NMR calculations with NWChem quantum chemistry program

I tried experimenting with some ab initio NMR calculations using the NWChem program. I used quinuclidine as a simple test case. The input file was:

echo
start quinuclidine_b3ylp_opt_nmr_shift

geometry
c         -6.06576        0.97412       -0.15301
c         -5.70979        1.98552       -1.28158
c         -4.36490        2.65888       -0.91745
h         -6.47615        2.78876       -1.35616
h         -5.68491        1.45114       -2.25601
c         -3.37563        1.54341       -0.50314
c         -3.86410        0.94013        0.84625
h         -2.33065        1.90335       -0.38494
h         -3.37977        0.77674       -1.30967
n         -5.29417        1.27496        1.08291
h         -3.26800        1.37762        1.67961
h         -3.67643       -0.15484        0.87417
h         -5.79345       -0.05430       -0.48418
h         -7.16227        0.96043        0.02635
c         -5.40220        2.73070        1.37047
c         -4.60680        3.55788        0.31857
h         -5.06760        2.98401        2.39936
h         -6.47931        3.01065        1.31755
h         -3.96835        3.24945       -1.77113
h         -3.61077        3.85719        0.71410
h         -5.16983        4.48719        0.08363
end

basis
* library 6-311G
end

dft
xc b3lyp
end

title "Quinuclidine B3LYP/6-311G geom opt nmr shift"

basis
* library 6-311G
end
dft
xc b3lyp
end

property
gshift
shielding
end



Next, I calculated the chemical shift of the TMS standard:

echo
start tms_b3lyp_opt_freq_nmr_shift

geometry units angstroms
si  -5.37335102864362      2.43869139121700     -0.00017138545369
c   -3.50689688966580      2.43868715601638     -0.00001133876668
c   -5.99529466705065      4.19430001974021      0.12209335653192
h   -5.64408778702798      4.67985919142380      1.04819971762123
h   -7.09781476514675      4.22936546250064      0.12467467866764
h   -5.64435465416627      4.80382888763710     -0.72767673129762
h   -3.10677010278341      2.89237170000804      0.92240456642084
h   -3.10633327621776      3.01074016686707     -0.85387463583008
h   -3.10622471891726      1.41321522592654     -0.06819713587793
c   -5.99568159818493      1.45555000740651      1.45925444723856
c   -5.99571944212990      1.66683194553091     -1.58150765777166
h   -5.64537419258706      1.88774171172852      2.41177722402156
h   -5.64408400205021      0.41082962309567      1.41778721376507
h   -7.09821437068132      1.43547142531899      1.48770835119311
h   -5.64584598058862      2.22683552653798     -2.46512226305553
h   -7.09824537965055      1.65038972286304     -1.61218398701376
h   -5.64368714450789      0.62658083618162     -1.68513442039297
end

basis
* library 6-311G
end

dft
xc b3lyp
end

title "TMS VWN5/aug-cc-pVDZ geom opt nmr shift"

basis
* library 6-311G
end
dft
xc b3lyp
end

property
gshift
shielding
end



However, the resulting chemical shifts are terribly off from the experimental ones. Am I doing an obvious mistake or is the big deviation just the best you can get out of such calculations? What is your experience and approach to ab initio NMR calculations (i.e. chemical shifts and coupling constants)?

• I also tried out calculation chemical shifts/coupling constants with ORCA. However, in all the simple test cases I tried out, I always got huge deviations from experiment. This is why I am unsure now if I make an obvious mistake, e.g. not using a solvent model or if those calculations just are that unprecise. I somehow feel that just not handling solvent or using an appropriately sized basis set should completely screw up the results of such calculations. – logical x 2 Nov 17 '16 at 21:12
• I know of some cases (such as Pt NMR) where the absolute isotropic shielding constant is EXTREMELY sensitive to the geometry (a deviation of 0.01 Angstroms can lead to a change in this value by a couple hundred ppm or more). However, you can sometimes get good error cancellation with your reference compound. Also, try considering solvent in your calculations (COSMO, PCM, etc.) though your mileage may vary. – LordStryker Nov 17 '16 at 21:57
• It is hard to discern what exactly went wrong without a peak at the output files. Did you scale the values output? See this site for more information: cheshirenmr.info/index.htm – BiggChemT Nov 18 '16 at 0:25
• Can you upload (or pastebin.com ) the relevant section of the outputs. I am curious about how they look like. I've never used NWChem nor Orca for chemical shift calculations. Thanks in advance. Also, notice that it can be important in order to answer your question. – user1420303 Nov 18 '16 at 0:48
• @user1420303 I will do that ASAP. – logical x 2 Nov 18 '16 at 8:09

I use Gaussian 09 to obtain NMR $\ce{^{13}C}$ chemical shifts (cs) with great accuracy. The best results are obtain if the calculated cs are scaled. So, I recommend to use this level of theory:
1. Opt => mPW1PW91/6-31G(d) with tight criteria (opt=tight)
2. NNR => mPW1PW91/6-31G(d) with nmr int=ultafine
3. To scale: $\text{scaled cs} = \text{(calculated cs)}*1.05 - 1.22$ (See reference)